1. Field of the Invention
The present invention relates to a liquid crystal display device having scanning lines and common lines and, more particularly, an LCD device having an improved structure for applying a common potential to common lines extending parallel to scanning lines.
2. Description of the Related Art
Liquid crystal display (LCD) devices of an active matrix driving mode (referred to as active matrix LCD devices hereinafter) are increasingly used as flat panel display devices for personal computers or television sets. Referring to FIG. 1A, a typical active matrix LCD device, generally designated by numeral 90, includes a TFT panel 92 defining a pixel array thereon and a counter panel 94 opposing the TFT panel 92, and liquid crystal disposed between the TFT panel 92 and the counter panel 94. The LCD device 90 also includes a plurality of vertical driver ICs 96 each mounted on a film tape for driving a plurality of scanning lines in the pixel array, and a plurality of data driver ICs each mounted on a film tape for driving a plurality of signal lines in the pixel array.
A conventional active matrix LCD device is described in Patent Publication JP-A-7-36061, for example. Referring to FIG. 1B, the TFT panel 70 of the conventional active matrix LCD device described therein includes a glass substrate or transparent substrate (not shown), a plurality of scanning lines 72 extending in a row direction on the glass substrate, a plurality of signal lines 74 extending in a column direction on the glass substrate, and an array of pixels disposed at respective intersections between the scanning lines 72 and the signal lines 74. A plurality of common lines 78 each extending parallel to a corresponding one of the scanning lines 72 are also disposed on the glass substrate. A silicon nitride film is interposed between the scanning lines 72 and the signal lines 74 for electric insulation.
Each pixel includes a thin film transistor (TFT) 76 as a switching element, a pixel electrode 80 connected to the source of the TFT 76, an LC element expressed as a capacitor 81 connected between the pixel electrode 80 and a counter electrode 83, and an additional capacitor element 82 connected between the pixel electrode 80 and the common line 78. The gate of the TFT 76 is connected to a corresponding scanning line 72, whereas the drain of the TFT 76 is connected to a corresponding signal line 74.
The proximal end of each scanning line 72 is connected to a scanning terminal 84 which is disposed for connection to a vertical driver IC mounted on a TCP (tape carrier package). Both the ends of each common line 78 is connected via respective through-holes 86 to respective overlying coupling lines 88A and 88B, which is disposed to apply a fixed potential or the ground potential equal to the potential of the counter electrode.
The scanning lines 72 and the common lines 78 are formed by patterning a single conductor layer. Each scanning line 72 intersects an overlying coupling line 88A, with a silicon nitride film interposed therebetween. The steps for forming the overlying coupling lines 88A and 88B and through-holes 86 for connecting the common lines 78 to the overlying coupling lines 88A and 88B complicate the fabrication process for the LCD device in the conventional technique.